1. . Field of the Invention
The present invention relates generally to screens for use in screening or filtering, such as in papermaking processes, wood pulp and other fibrous or particulate fluid suspensions for removing foreign particles therefrom, and more particularly, to a screen media and a screening passage for a screen media.
2. Description of the Relevant Art
Most paper today is made on Fourdrinier machines patterned after the first successful papermaking machine, which was developed in the early years of the 19th century. The heart of the Fourdrinier machine is an endless belt of wire mesh that moves horizontally over a number of rolls. A flow of watery paper pulp from a head box at the beginning of the papermaking machine is spread on the level moving belt. Water passing through the belt is collected and is remixed with the pulp to salvage the fiber contained in it. Spreading of the sheet of wet pulp on the wire belt is limited by rubber deckle straps moving at the sides of the belt. Suction pumps beneath the belt hasten drying of the paper, and the belt itself is agitated from side to side to aid the felting of the paper fibers. As the paper travels along the belt it passes under a rotating cylinder called a dandy roll. The surface of this cylinder is covered with wire mesh or single wires to impart a wove or laid surface to the paper. In addition, the surface may carry words or patterns worked in wire; these are impressed in the paper and appear as watermarks that identify the grade of paper and the maker.
Near the far end of the machine, the belt passes through two felt-covered couch rolls. These rolls press still more water out of the fibrous web and consolidate the fiber, giving the paper enough strength to continue through the machine without the support of the belt. From the couch rolls, the paper is carried on a belt of cloth through two sets of smooth metal press rolls. These rolls impart a smooth finish to the upper and lower surface of the paper. After pressing, the paper is fully formed. Thereafter, the paper is carried through a series of heated rolls that complete the drying. The next step is calendering, pressing between smooth chilled rolls to impart on the paper a smooth finish known as a machine finish.
The first step in machine papermaking is thus the preparation of the pulp from raw material. The raw materials chiefly used in modern papermaking are cotton or linen rags and wood pulp. Today more than 95 percent of paper is made from wood cellulose. For the cheapest grades of paper, such as newsprint, ground wood pulp alone is used; for better grades, chemical wood pulp, or a mixture of pulp and rag fiber is employed; and for the finest papers, such as the highest grades of writing papers, rag fiber alone is used.
There are several processes for the preparation of fibers from rags, wood and combinations thereof. In each process, several filtering steps are required for separating useable fiber from unusable fiber and contaminants. For example, a typical preparation operation employs three stages of filtering, or screening. However, depending on the starting material and/or the desired purity and composition of fibers, more or less stages of screening may be employed. Also, upstream processing to prepare raw fiber material may employ one or more screening operations. The screens used in these upstream operations are often referred to as the "broke" screens where raw material, such as wood cellulose, recycled paper, rags and the like are broken down into fibers.
Initially, stock comprising a slurry of about a 1-4 percent raw fiber material and the balance water composition is prepared. The stock is passed through each of the screening stages to remove contaminants such as plastic, sand, grit, sheaves, splinters, rocks and the like from the stock and leave a usable fiber and water pulp slurry for use in the paper making process.
The screening stages may be arranged as a series of flat screens; however, it is more typical to employ cylinders constructed of screen media. The pulp slurry may be arranged to flow from the outside of the cylinder inward. A more common arrangement is for the pulp slurry to flow from the inside of the cylinder outward. A rotor or other device is generally incorporated into the screen stage. The rotor creates pressure pulses for moving the pulp slurry through the screen media and provides a self-cleaning function.
The majority of screen media for the paper making, pulping processes currently use screens containing parallel filter passages or orifices through which slurry material to be filtered passes. The passages are primarily perpendicular to two parallel planes or sides of screen material defining the inflow and outflow side of the screen media. There are three primary characteristics of the screen media that tend to work against each other: capacity (the throughput of stock), efficiency (the percentage of contaminants filtered out) and runability (the tendency of the screen to blind, mat, or plug). Thus, the designers of the screen media must account for each of these characteristics. Additional requirements for the screen media include sufficient structure on both sides of the passages to prevent breaking of the beams, the screen material between the passages, and to prevent the beams from bending, or warping, which can result in increased, and hence, improper passage size. The screen must also provide a filter slit within each passage each of specific width, typically between about 0.05 millimeter (mm) to 0.7 mm, with a maximum allowable variation of about +/-0.025 mm to optimize capacity, efficiency, and runability.
To manufacture a screen, a metal plate, typically made stainless steel, thicker than the required final screen thickness is prepared to the appropriate dimensions. For each passage in the screen, a groove, known as the back groove, is cut from what will become the outflow side of the screen. On the inflow side of the screen, a contour cut is made, substantially in alignment with the back groove. The cut depths of the back groove and the contour cut at a center portion thereof leave a proper amount of residual material into which thereafter a slit cut is made. The slit cut results in the creation of a filter slit completely through the residual material through which the slurry to be filtered actually passes during operation. The cut that forms the slit is shorter in length than both the back groove and the contour cut. The fabrication process is repeated for each of the passages to be formed in the screen. When machining is completed, the resultant filter screen may be used as is as a flat screen, or it may rolled or otherwise formed to provide a curvature for use in a cylinder screen.
The most prevalent fabrication technique uses a horizontal mill and a 70 mm milling cutter for each of the above-described cuts. This technique, however, suffers numerous disadvantages as will be described.
A chief drawback of the several drawbacks of milling filter passages in this manner is a resulting limited effective slit length, which is a measure of the length of the slit through which filtering actually takes place, that is less than the actual length of the slit. The horizontal mill uses a radiused cutting tool. While the thickness of the residual plate material near the center portion of the slit can be suitably thin, the residual material becomes substantially thicker at each end and hence is not of uniform thickness. As a result, a portion at each end of the slit does not extend completely through the residual material which significantly shortens the effective slit length to a distance that is less than the actual length of the slit, reducing capacity. Where the slit does extend through the thicker portion of the residual material, the flow of material to be filtered at each end of the slit is greatly reduced or even can be completely be obstructed depending upon slurry boundary layer conditions further reducing capacity. Hence, the effective slit length for a given back groove or contour cut length is substantially reduced thus reducing screen capacity. This unusable portion of the back groove or contour cut where the residual material is thickest along the slit is often referred to as the end margin.
Additional drawbacks of milling include increased stress cracking at the ends of the back groove resulting from the sharp intersection of the slit to the back groove. A reduced amount of material at the back groove slit intersection at the ends also results in localized reduced rigidity of the screen media. Increased processing is also required to remove burrs inherent to the milling process and aggravated by having to use a highly machinable material, such as, for example, 316L or 316 resulphurized steel. There is also limited ability to control slit width because a minimum cutter thickness is required to reduce cutter breakage and to prevent cutter walk, i.e., to preserve straightness while cutting the thicker material. Finally, the overall process results in a less than ideal surface finish.
Because of the foregoing process limitations, subsequent time consuming and costly manual deburring using hand held blades, buffers and polishers is required after milling. Manual deburring often results in improper edge and corner radii and may exacerbate stress cracking problems. Additional operations also include blasting using water or air and an abrasive, electropolishing and plating to obtain a suitable surface finish. These operations can adversely effect slit width, i.e., blasting and polishing may undesirably increase width while plating may undesirably narrow width. All additional processing obviously increases process complexity and product cost.
Thus, there is a need for a screen media that provides enhanced capacity, efficiency and runability yet which is easily fabricated. There is also a need for a screen media that provides increased capacity without adversely effecting efficiency and runability. There is still further a need for a screen media where the resultant effective slit length is about the same as slit length increasing filter capacity.